For the past fifty years, ferrimagnetic oxide ceramics (ferrites) have been prized for a range of properties that has no equivalent in the existing metal magnetic materials. These ferritic materials are discussed, e.g., in an article by Alex Goldman entitled "Magnetic Ceramics" which appears on pages 147-189 of Lionel M. Levinson's "Electronic Ceramics" (Marcel Dekker, Inc., New York, 1988).
At page 170 of the Goldman article, it is taught that ferritic materials may be used to produce permanent magnet materials, cores for high-frequency power supplies, cores for low-level indictors for telecommunications filters, deflection yokes for television, high-frequency transformers, recording heads for audio and digital magnetic recording, recording media, radar absorbing paint, sensors, microwave components, copier powders, electrodes, ferrofluids, and magnetostrictive transducers.
One of the more important applications for ferritic materials is in microwave technology, where these materials may be used to produce ferrite loaded rectangular waveguides, junction circulators, magnetically tunable microwave filters, microwave switches, and the like. See, e.g., J. Helszain's "Principles of Mircowave Ferrite Engineering"(Wiley-Interscience, New York, 1969).
Films of ferritic material, with thicknesses of from about 0.1 to about 500 microns, have been proposed for use in electronic circuits; see, for example, "Advances in Ceramics, Volume 16, Fourth International Conference on Ferrites, Part II," Edited by Franklin F. Y. Wang (The American Ceramic Society, Inc., Columbus, Ohio, 1984). These ferritic films are useful for microwave integrated circuits, mangetooptical devices, microinductance devices, and the like.
To the best of applicants' knowledge, an economical, reliable process for the large-scale production of high-quality ferritic films has not been described in the prior art. Thus, for example, on page 9 of Volume 15 of "Advances in Ceramics, Fourth International Conference on Ferrites, Part I," Edited by Franklin F. Y. Wang (The American Ceramic Society, Inc., Columbus, Ohio, 1984), it was disclosed that "Thin films of ferrite . . . have been produced experimentally . . . . but a manufacturing technology to produce a high-quality product has not been established."
In 1971, in U.S. Pat. No. 3,576,672, Douglas H. Harris and Richard J. Janowiecki described a process for depositing ferrite coatings by using a direct current arc plasma generator to heat powdered ferrites. Although this process was superior to prior art processes, it often produced ferrite coatings which were substantially inhomogeneous.
In 1989, in U.S. Pat. No. 4,816,292, Hamime Machida described a process for applying a ferrite coating upon a substrate in which a metal-ion solution is applied to the substrate and the ferrous ion therein is thereafter oxidized. The Machida process often requires the substrate used in it to be heated (see column 3) and, despite such heating, often produces a ferrite film layer with insufficient crystallization (see lines 67 et seq. of column 3), in which case the film must be subjected to a subsequent annealing (see column 4). Because of the need for heating the substrate and/or annealing the ferrite coating, the Machida process is not suitable for the large-scale production of ferrite coatings.
Another ferrite coating process is described in U.S. Pat. No. 4,837,046 of Massao Oishi et al. At column 1 of this Oishi patent, the disadvantages of several prior ferrite coating processes are discussed. At column 1 of the patent, Oishi teaches that these prior processes often produce a product whose magnetic recording density is low, and/or require high temperatures, and/or cannot utilize as a substrate a material having a low melting point or a low decomposition temperature or poor heat stability, and/or do not achieve a satisfactory rate of production in an industrial scale, and/or do not produce a suitably high-quality product. However, Oishi's process also suffers from several disadvantages. In the first place, it only can be used on a relatively limited range of substrates (see column 4). In the second place, because Oishi's process usually requires the rotation of the substrate at a speed of about 400 revolutions per minute (see Example 1), a inhomogeneous coating often produced with substantial differences between its center and edge. In the third place, a process such as Oishi's which requires rapid rotation of a substrate is not suitable for large scale production of coatings.
In 1990, another attempt was made to provide a commercially feasible process for producing ferritic coatings. This attempt was described in U.S. Pat. No. 4,948,626 of Yasunaga et al. The Yasunaga process, however, required the use of a vacuum chamber and, obviously, was also not suitable for the large scale production of ferritic coatings. A substantial amount of energy, time, and money is required for a vacuum system and its operation. In addition, the size of the film which can be made by the vacuum deposition processes is limited by the size of the vacuum chamber used.
It is an object of this invention to provide a process for the production of ferritic coatings which may be conducted under atmospheric conditions.
It is another object of this invention to provide a process for the production of ferritic coatings which does not require that the substrate used in the process be heated.
It is yet another object of this invention to provide a process for the production of ferritic coatings which does not require that the coating applied to the substrate material be annealed after deposition.
It is yet another object of this invention to provide a process for the production of ferritic coatings which produces a coating which is substantially homogeneous.
It is yet another object of this invention to provide a process for the production of ferritic coatings which is suitable for the large-scale production of such coatings.
It is yet another object of this invention to provide a process for the production of ferritic coatings which can be used to produce complex, coated shaped articles.